We model a population of \(N\)  individuals engaged in pairwise social interactions with their neighbors. Each player engages with each of her neighbors in a simple social dilemma called the donation game [1]}, choosing either to cooperate (\(C\) ) or defect (\(D\) ). A cooperative act means paying a cost \(c\)  to provide her opponent with a benefit \(b\) . In general, selection can favor cooperation in structured populations provided the benefit-to-cost ratio \(b/c\)  is sufficiently large [2]}, [3]}. Here, we analyze how the critical benefit-to-cost ratio required to support cooperation depends upon directionality in the structure of social interactions.
